Field of the Invention
The present invention relates to a conductive polymer material and a substrate having a conductive film formed thereon by using the conductive polymer material.
Description of the Related Art
A polymer having a conjugated double bond (i.e. π-conjugated polymer) does not show a conductivity by itself; however, if an appropriate anionic molecule is doped therein, it can express a conductivity, thereby giving a conductive polymer material (i.e. conductive polymer composition). As to the π-conjugated polymer, polyacetylene; (hetero) aromatic polymers such as polythiophene, polyselenophene, polytellurophene, polypyrrole, and polyaniline; a mixture thereof, etc., are used; and as to the anionic molecule (dopant), an anion of sulfonic acid type is most commonly used. This is because a sulfonic acid, which is a strong acid, can efficiently interact with the aforementioned π-conjugated polymers.
As to the anionic dopant of sulfonic acid type, sulfonic acid polymers such as polyvinyl sulfonic acid and polystyrene sulfonic acid (PSS) are widely used (Patent Document 1). The sulfonic acid polymer includes a vinylperfluoroalkyl ether sulfonic acid typified by Nafion (registered trademark), which is used for a fuel cell.
Polystyrene sulfonic acid (PSS), which is a homopolymer of a sulfonic acid, has a sulfonic acid as a repeated monomer unit in the polymer main chain, so that it has a high doping effect to the π-conjugated polymer, and also can enhance water dispersibility of the π-conjugated polymer after being doped. This is because the hydrophilicity is kept due to the sulfo groups excessively present in PSS, and the dispersibility into water is therefore enhanced dramatically.
Polythiophene having PSS as a dopant exhibits high conductivity and can be handled as an aqueous dispersion, so that it is expected to be used as a coating-type conductive film material in place of ITO (indium-tin oxide). As mentioned above, however, PSS is a water-soluble resin, and is hardly soluble in an organic solvent. Accordingly, the polythiophene having PSS as a dopant also has a high hydrophilicity, but a low affinity to an organic solvent and an organic substrate, and thus, it is difficult to disperse it into an organic solvent or to form a film onto an organic substrate.
Besides, when the polythiophene having PSS as a dopant is used in, for example, a conductive film for an organic EL lighting, a large quantity of water tends to remain in the conductive film and the conductive film thus formed tends to absorb moisture from an outside atmosphere since the polythiophene having PSS as a dopant has an extremely high hydrophilicity as mentioned above. As a result, the problems arise that the luminous body of the organic EL chemically changes, thereby the light emitting capability is deteriorated, and that water agglomerates with the passage of time and defects are caused, which results in shortening of the lifetime of the whole organic EL device. Furthermore, there arise other problems in the polythiophene having PSS as a dopant that particles in the aqueous dispersion becomes large, the film surface becomes rough after the film formation, and a non-light emitting region, called dark spot, is caused when used for the organic EL lighting.
In addition, since the polythiophene having PSS as a dopant has an absorption at a wavelength of about 500 nm in the blue region, in the case that this material is used as a film coating a transparent substrate such as a transparent electrode, there arises another problem that when the conductivity required for the device to function is made up by the solid concentration or the thickness of the film, transmittance of the film is affected.
Furthermore, with respect to quantitative relation between the π-conjugated polymer and the dopant polymer in the polythiophene having PSS as a dopant, the molar quantity of sulfo groups in PSS exceeds the molar quantity of thiophene. Such excess sulfo group having high hydrophilicity allows the conductive composite to disperse in water. Accordingly, aqueous dispersions of a conductive polymer are strongly acidic. However, strongly acidic aqueous solutions are highly corrosive to metals, and therefore they have to be handled with caution.
In order to neutralize the strongly acidic aqueous solutions, there has been proposed methods to add a basic compound as a conductive material composition (Patent Documents 2 and 3). These documents disclose an addition of a basic compound having an amino group is exemplified.
Moreover, Patent Document 4 discloses a conductive polymer composition composed of a conductive polymer which contains a π-conjugated polymer formed of a repeating unit selected from thiophene, selenophene, tellurophene, pyrrole, aniline, and a polycyclic aromatic compound, and a fluorinated acid polymer which can be wetted by an organic solvent and 50% or more of which is neutralized by a cation. There is mentioned alkaline metals such as lithium and sodium, and amine compounds are exemplified as a cation.
In the neutralization by the foregoing cations or amine compounds, although it is possible to neutralize the aqueous solutions, the problem arises that the conductivity is lowered. Accordingly, it is desired to develop a conductive material which provides a neutral solution without lowering the conductivity.
Moreover, in the aqueous dispersion of polythiophene having PSS as a dopant, the particles are agglomerated. After the polymerization of the composite of the polythiophene having PSS as a dopant, the particles need to be pulverized by a disperser, however, the particles are getting larger with the passage of time. This is attributed to the growth of the agglomerate due to ionic bond between the particles of PSS-polythiophene composite. If the particles are getting larger, a striation occurs when the conductive solution is applied by methods such as spin coating, and a flat film cannot be obtained, which causes a dark spot when it is applied to an organic EL illumination. Accordingly, it is desired to develop a conductive solution material in which the agglomeration does not occur with the passage of time.
Moreover, the polythiophene having PSS as a dopant can also be used as a hole injection layer. In this case, the hole injection layer is provided between a transparent electrode such as ITO and a light-emitting layer. The hole injection layer does not require high conductivity since the under transparent electrode ensures the conductivity. For the hole injection layer, no occurrence of dark spot and high hole-transporting ability are required.